This invention relates to a method and apparatus for coating a moving web with one or a plurality of coatings and more particularly to a method and apparatus for coating a moving web with one or more liquid compositions which are subsequently dried, set or gelled on the web.
Presently, there is available coating apparatus for applying a plurality of superimposed layers to a moving web. Typical apparatus utilizes a plurality of inclined surfaces separated by exit slots through each of which is metered a coating solution onto an adjacent inclined surface. The coating solutions flow by gravity over the inclined surfaces and those metered through upstream slots flow over coating solutions metered through downstream slots and form a multilayered stream, formed of the individual coating solutions that cascade over the downstream inclined surfaces. As the coating solutions flow under gravity over the inclined surfaces, each layer becomes smooth and is of uniform thickness. At the last inclined surface or slide, the multilayered stream is stratified in a configuration which constitutes the desired multilayered coating to be contacted with the moving web. The end of the last slide is spaced apart from the moving web so that the multilayered stream exiting the last slide toward the web forms a bead or bridge between the last slide and the moving web. A pressure differential generally is effected across the bead by applying a vacuum to the bottom surface of the bead immediately adjacent the top surface of the web to stabilize the bead against excessive vibration and rupture. As the web contacts the bead, it entrains the multilayered coating, thereby becoming coated.
The coating apparatus described is especially useful, for example, to form webs coated with superimposed layers of aqueous photographic compositions including light sensitive materials, chemical sensitizers, antifoggants, developing agents and the like. These compositions are mixed with synthetic or naturally occurring colloids such as gelatin, polyvinyl compounds, or the like, which form non-flowing set layers containing the photographic compositions when the colloid is dried on the web.
Typical examples of the apparatus described above are described in U.S. Pat. Nos. 2,761,419, 3,220,877, 3,749,053, 3,928,678 and 3,928,679. In the apparatus disclosed in U.S. Pat. Nos. 2,761,419 and 3,220,877, the angle between the last slide and the tangent to the web at the point of coating contact is acute. In the apparatus disclosed in U.S. Pat. Nos. 3,928,678 and 3,928,679, the angle between the last slide and the tangent to the web at the point of coating contact may be from 45 degrees to 125 degrees. While the apparatus described in these patents permit adequate coating rates when the lowermost layer is of low viscosity of about 35 centipoise or less, the rate of applying multilayered coatings of the desired thickness on the web is undesirably limited when the lowermost layer has a relatively high viscosity of about 100 to 200 centipoise or more. In order to maintain stability of the bead between the web and the last slide and to prevent turbulence leading to intermixing of the layers, an increased vacuum on the lowermost layer of the bead must be applied when increasing coating rates. When applying the coating at an acute angle or at a right angle in the manner shown in the above-identified patents, longitudinal ribbing of multilayered coating in the bead occurs when applying a relatively low pressure differential on the bead so that the coating applied to the web is non-uniform in the lateral web direction. Thus, the operator is forced to coat the web at relatively low rates in order to avoid forming unacceptable coated webs.
In the apparatus described in U.S. Pat. No. 3,749,053, a planar web is passed through a vacuum chamber and past a slide coater positioned adjacent the chamber outlet, which coater is adapted to coat the web with a multilayer liquid composition. The angle between the plane of the moving web and the slide surface is obtuse. This apparatus is undesirable as a means for coating webs since the angle between the slide surface and the moving web is fixed so that it cannot be adjusted to provide optimum coating rates for liquids of varying viscosities.